A prior art image reading apparatus is generally provided with a light emitting diode (LED) as its light source. Recently a more compact image reading apparatus has been proposed, in which a solid state light source such as an electroluminescent (EL) device has been substituted for a light emitting diode (LED). The electroluminescent device used as a light emitting device has been combined with a photosensing device into an integral unit, i.e. an integral light emitting and sensing device, to save space.
FIGS. 6 and 7 show a prior art image reading apparatus of a type having an integral light emitting and sensing device as stated above. It will be observed that the image reading apparatus includes a photoreceptor array 30 comprising an array of photoreceptor elements 31. An electroluminescent device 40 is disposed immediately above the array 30 separated from it by an adhesive layer 50, and provided with optically transmissive areas 60. Each of the optically transmissive area 60 directly overlies a photoreceptor 31. Light emitted from the electroluminescent device 40 is perpendicularly incident upon an original 70, so that uniform illumination can be obtained. Reflected light 80 from the original 70 passes through the optically transmissive area 60 onto the photoreceptors 31. The optical path length of reflected light 80 from the original 70 to the photoreceptors 31 is consequently minimized.
The optically transmissive area 60 of the electroluminescent device 40 is configured in the following manner. A transparent electrode 42, an insulating layer 43, a luminescent layer 44, and an insulating layer 45 are successively deposited on a transparent substrate 41 by a thin film technique known in the art. A light shielding electrode 46 is then deposited and patterned by etching such that rectangular openings 46a are formed therein. Since optically transparent materials are used for the transparent electrode 42, the insulating layers 43 and 45, and the luminescent layer 44, portions of these layers directly overlying the openings 46a formed in the light shielding electrode 46 function as the optically transmissive areas 60.
This configuration of the prior art electroluminescent device, however, requires the electroluminescent device be made of thin films, which entails high manufacturing cost and involves a difficulty in manufacturing a large device. The range of realizable size of a resulting device is limited depending on the size of vacuum chamber used for the thin film process.
On the other hand, there is another type of electroluminescent device in the art, in which a luminescent layer is formed by a thick film technique such as screen printing. Whereas this thick film type electroluminescent device is free from the foregoing problem and difficulty, it suffers from another problem, that of relatively low transmission efficiency. The luminescent layer contains fluorescent particles such as ZnS:(Cu, Al) which are dispersed in an organic binder such as cyanoethylpolyvinylalcohol (CEPVA). The fluorescent particles and the organic binder have their own refractive indexes, which differ from each other. Difference in refractive index causes reflected light from an original to be scattered in the luminescent layer, thus resulting in relatively low transmission efficiency. If an integral light emitting and sensing device is constructed with a thick film type electroluminescent device, portions of the luminescent layer right above the photoreceptors must be removed in order to keep reflected light from scattering. Nevertheless, the luminescent layer has poor resistance to etching, because the organic binder contained in the luminescent layer has high permeability and absorption of water, and a high solubility in organic solvents. In addition to that, it is difficult to form very fine patterns in the luminescent layer, because it is relatively thick, 10 .mu.m to 100 .mu.m. In consequence, it is difficult to introduce a thick film type electroluminescent device into a conventional image reading apparatus using a similar configuration to that described above.